This invention relates in general to peristaltic pumps and more particularly to implantable peristaltic pumps for infusing treatments into an organism.
Peristaltic pumps find general application in the medical field. They operate on the principle that a flexible tube provided with advancing occluded portions, usually caused by rotating rollers held against the tube periphery, can be used to pump fluid from one location to another. These pumps incorporate coplanar geometry in which the pump rollers orbit within the plane defined by the pump tube, which is held in a stationary race. Peristaltic pumps that are exemplary of the prior art are disclosed in U.S. Pat. No. 4,692,147 (Duggan) and U.S. Pat. No. 4,576,556 (Thompson), both assigned to Medtronic, Inc. of Minneapolis, Minn. The subject matter of these patents is incorporated herein by reference.
One problem associated with the prior art pumps is that they typically require a great deal of effort and expense in their assembly and maintenance in order to closely control the tolerances relating to the tube alignment and the occluding force applied by the rollers to various portions of the tube. For example, Duggan, while providing a highly accurate peristaltic pump, requires close tolerances and accurate machining of some pump components, which adds considerably to manufacturing expense. The pump tube described by Duggan is extruded with flanges or wings protruding from the outer wall of the tube. These features help insure that the pump tube remains aligned in a race. However, the presence of the flange increases costs by complicating the tube fabrication process. The pump race requires additional depth to accommodate the wings and adds to the inherent thickness of the pump assembly. The pump described by Duggan requires an alignment procedure that has proven to be time consuming and expensive.
Another problem with prior art pumps involves the control of the occlusion force applied to the pump tube. Often, a pressurized reservoir is provided in communication with the pump tube inlet to facilitate the advance of the pumped fluid. It is desirable to provide sufficient occluding force on the pump tube to prevent reservoir-pressure induced leakage through the occluded portion of the pump tube, which might be catastrophic. Frequently, prior art pumps are assembled in a significantly over-occluded state to ensure sufficient occlusion forces at all points on the pump tube. On the other hand, over-occlusion--excessive compressive load on the tube--is undesirable because it results in unnecessary friction and leads to increased wear and excessive power consumption by the pump drive system. Excessive power consumption is particularly undesirable in the field of implantable, battery-powered pumps. Over-occlusion results in increased cyclic loading of the pump tube and thus reduces the tube leakage safety margin and useful life. Similarly, over-occlusion results in unnecessary wear on the pump rollers, bearings and other components. In the past, much time and effort has been expended to address these competing factors.
The level of occluding force needed to prevent leakage past the occluded portion of the tube is a function, in part, of the inlet pressure on the pump tube. In implantable pumps, a pressurized reservoir is provided in fluid communication with the pump tube to supply treatment to the pump inlet. The reservoir is charged with a propellant as well as the treatment. As the reservoir is emptied, pressure will decrease and the amount of occluding force necessary to prevent leakage through the occluded portions of the pump tube is reduced. Theoretically, then, the torque and therefore the power required to operate the pump rotor could be reduced as the pump inlet pressure decreases. Prior art pumps, however, have provided no means to take advantage of this principle, leading to unnecessary expenditure of pump power and over occlusion of the pump tube.
It is known to provide pump tubes with shims at various locations on the tube periphery to control the occluding force. Shims may take the form of a silicone rubber sheet that is fit between the rollers and the tube. Shims are provided in varying thicknesses, depending on the gap between the pump roller surfaces and the pump race at varying points along the race. Shims are selectively matched to various parts of the pump tube to compensate for variances in tube, pump housing and race dimensions and to achieve the optimum occlusion force at all points on the pump tube. Each individual pump has to be inspected and measured to determine the proper shim thicknesses to be applied at appropriate points along the tube. This has resulted the expenditure of considerable time and effort in pump manufacturing in the past.
It is also known, as exemplified in the implantable device of Thompson, to provide spring-biased rollers on the pump rotor in order to provide a resilient force outward against the tube. Such a passive spring configuration, however, has only a limited ability to control the occluding force applied to the pump tube.